Automated testing for cellular telephone system including emergency positioning

ABSTRACT

A method for automatically testing cellular telephone equipment includes a method for determining a geographic location of a mobile unit. The system includes monitoring sites located, e.g., at high elevations, so that each monitoring site electronically covers a geographical area including several cellular telephone base stations. Locations of mobile cellular stations, especially of such stations placing emergency 911 calls, are determined by comparing signal time-of-reception and other observable signal parameters at a combination of three cell sites and/or monitoring sites. Testing functions include transmitting gradually increasing power levels on a frequency assigned to a particular base station to determine the power level required to acquire service from that base station. Periodic repetitions are monitored over time to indicate any changes or degradation in performance. A scanner scans a designated group of control-channel frequencies, selecting active channels for measurement of transmission parameters and detection of anomalous transmission characteristics on any particular channel.

CROSS REFERENCE TO RELATED APPLICATIONS

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and apparatus for determining ageographic position of a mobile telephone station, in combination withautomated testing of cellular telephone equipment; and in particular tosuch methods and apparatus including elevated monitoring sites foractive and passive testing of cellular base stations and mobilestations.

2. Description of the Related Art

Self-testing capabilities of cellular telephone sites currently includestest functions such as monitoring antenna performance by measuring thevoltage standing wave ratio (VSWR), and monitoring site controllermalfunctions and environmental conditions such as air conditioning andpower failures. Generally, self-testing of electronic equipment detectsrelatively major malfunctions such as failures of individual radios orentire cell sites.

Currently, self-testing does not include remote monitoring oftransmitted power levels to ensure that the-entire signal path throughthe antenna is working properly and that the antenna is efficientlyradiating the power delivered to it. Use of test mobiles at a site isknown, but these are used to detect major failures, i.e., a problem isdetected if the test mobiles do not respond when called. What is neededis an automated system which tests a complete end-to-end signal path,and which tests all the channels (frequencies, time slots, or PN codes)assigned to a particular cell site so that the entire system, includingaudio paths, is tested. Such a test system would not only detectequipment failures, but also detect more subtle changes in timingparameters, operational parameters, and system settings.

Such a testing system, properly designed, can also determine thelocation of cellular mobile stations in emergency situations. Thefamiliar “911” is widely used as an emergency telephone number; 911requests are relayed to the proper emergency-services department forresponse. The effectiveness of emergency services depends, of course, onemergency personnel being able to get to the caller without undue delay.To this end, the U.S. government has recently promulgated regulationsrequiring that providers of cellular telephone service be able to supplyinformation on the position of a mobile station making an emergencycall.

A problem with responding to requests for emergency assistance is thatpersons using mobile telephones often are unable to give their exactlocation when making an emergency call. A caller who happens to be inunfamiliar terrain does not know local landmarks, and may not know thename of the street or road on which he is driving. This lack ofinformation regarding the caller's location hampers and delays effortsto get emergency assistance to the caller, whether the request is formedical assistance, the police, or roadside repairs.

The geographic size and shape of each cell in a cellular telephonenetwork is largely determined by the coverage of transmit/receiveantennas located at the central cell site, and by the surroundingterrain. In open country, cells are substantially circular, withoverlapping borders. Where buildings or uneven topography blockradio-frequency transmissions, the cells may be quite irregular inshape.

A mobile telephone station is handed off from cell to cell based on thereceived signal strength and signal quality at each cell site, with thecall being routed through the cell receiving or providing the bestsignal. In a metropolitan area, a number of cells may be capable ofreceiving a 911 emergency call from a particular mobile station.Triangulation, using the signal time-of-arrival at three or more sites,angle of arrival, or some combination of observable signal parameters,may be used to determine the position of such a caller. In rural areas,however, cell sites are generally too widely spaced for triangulation tobe possible. This gives rise to situations in which a caller makes anemergency 911 call, is asked his location by the dispatcher, and isunable to give accurate information. Emergency services are thusdelayed, sometimes with serious consequences. What is needed, thereforeis a system combining automated testing functions with the ability todetermine a geographic position of a mobile station.

SUMMARY OF THE INVENTION

Parameters of cellular system performance, both base stations and mobilestations, are measured at automated monitoring sites. At least some ofthe monitoring sites are located at high-elevation sites to give alarge-radius radio horizon, and/or at existing cell sites. Amicroprocessor, under either local or remote control, is placed at eachmonitoring site. The microprocessor controls a number of “test mobiles;”the test mobiles are interconnected to transmit/receive antennas. Poweroutput of the test mobiles can be varied by the control microprocessor;the test mobiles are also capable of transmitting without the usual callset-up process, “creatively” violating their usual protocol in order totest system functions.

By slowly increasing or decreasing transmitted power on a particularfrequency, the monitoring site can periodically test the power levelrequired to acquire service on each channel served by a cell site, andthe power setpoints or “dynamic power control” properties the siteexhibits. Hand-off performance between cells is tested by increasingpower on one test mobile and decreasing it on another at a differentlocation, and recording the levels at which the hand-off occurs.Advanced protocols such as IS-136 and IS-95 require the test mobile tomanipulate its reporting of signal parameters received from a servingsite, but handoff testing may be accomplished in a conceptually similarway.

Recording time-of-arrival at several monitoring locations of a signalfrom a mobile station making an emergency 911 call provides data fordetermining a position solution for the mobile station. The presentinvention determines TDOA (time difference of arrival) by separating areceived signal into its component frequencies, and plotting phasedifferences for individual frequencies at different receiving sites,either cell sites or monitoring sites. A slope of the phase-shift plotis proportional to the TDOA at different sites. The arrival times thusdetermined are used to plot a geographic location of the mobile stationgenerating the call.

Data processing for the positioning functioning is done by a centralE-911 computer, which may also be linked to the mobile telephoneswitching office (MTSO) computer. Occasionally data will be availablefrom only the serving cell site and one other site, in which case themobile's position can be narrowed to two possible choices. Comparison ofthe two locations to a map showing locations of roads, etc., can ofteneliminate one of the positions. Data from a combination of three sites,either cell sites or monitoring-only sites, using a time-of-arrivalalgorithm, will give a specific geographic location of the mobilestation.

The same transmitter-locating technology may also be used for“value-added” services such as fleet vehicle tracking, non-emergencymotorist assistance, etc., limited only by the imaginations of serviceproviders. Thus for the purposes of this application, “911 calls” mayinclude non-emergency calls or mobile registration actions which alsoare used to generate location fixes.

Based on the above, it is an object of this invention to provide amethod by which operational parameters of cellular telephone sites maybe monitored on an ongoing basis.

Another object is to provide a combination of a testing and monitoringsystem which further uses time data and/or other observable signalparameters collected by a monitoring site and by cellular base stationsto automatically provide an accurate geographic location of a mobilestation placing an emergency 911 call, or to monitor location of certainmobile stations on a continuing basis, utilizing routine transmissionsincluding registration and/or non-emergency transmissions.

A further object of the invention is to provide such a method which isautomated, and which periodically tests the complete signal path orchannel for each cellular frequency served by a particular site.

Another object is to provide a method by which several cellular sitesmay be monitored and tested by a single monitoring site located at ahigh elevation.

The specific nature of the invention, as well as other objects, uses,and advantages thereof, will clearly appear from the followingdescription and from the accompanying drawings, the different views ofwhich are not necessarily scale drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a geographical area including three monitoringsites and their respective radio horizons, within which are includedseveral cellular base stations and a variable number of cellular mobilestations.

FIG. 2 is a schematic view of mobile telephone stations within an urbanarea, in which cell sites are placed more closely together.

FIG. 3 is a schematic view of mobile stations in a rural area, in whichcell sites are placed as far apart as practicable, and some cell sitecoverage gaps exist.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 shows a map view of a geographicalarea including three remote cellular monitoring sites 10. As shown inthe drawing, the radio horizons of the monitoring sites are much largerthan the radio horizons of the cellular base stations 12. Located ontall towers or other suitably elevated locations such as mountains, tallbuildings, etc., the monitoring sites are placed to receive RFtransmissions from a number of base stations. Cellular telephone basestations are commonly referred to as “cell sites.” The monitoring sitesalso receive transmissions from cellular telephone mobile stations 14within their radio horizon. A mobile station 14 is defined as a cellulartelephone station, either mobile, portable, or hand-held, used by aperson on foot or in any type of vehicle, including ground vehicles,boats and other marine vessels, and aircraft. “Cellular telephonesystem,” as used herein, is defined as including PCS systems; althoughPCS frequencies are generally different from cellular frequencies, somePCS and cellular telephone systems are transparent to one another.

The monitoring sites 10 may be thought of as data collection centers;they operate both in an active mode and in a passive mode, collecting avariety of data regarding cell site performance, and also data which isused to determine position solutions for particular mobile stations.

Referring to FIG. 2, a mobile station 14 placing a call usually iswithin range of cell sites 12 in an urban area. This is frequently truein urban areas, but in rural areas (FIG. 3) a signal from a mobilestation will be received at fewer cell sites, perhaps only one. A callfrom a mobile station is carried by a particular cell site based on thereceived signal strength at/from the various cell sites receiving thesignal (or on other signal quality measures, depending upon the protocolused); the call will be routed through the site(s) receiving thestrongest or best quality signal. Of course, changes in the mobilestation's position will cause the mobile station's signal level andsignal quality to change at the various cell sites within range. Whenthe signal level at (or from) another cell site becomes stronger or ofbetter quality than that at the original cell site, the routing of thecall is switched or “handed off” to the site receiving or providing thestronger or higher-quality signal. Such a hand-off is automatic and sobrief that a caller is usually unaware when it happens. The precedingdescription applies to cellular systems in general, whether the mobileunits are in use by pedestrians, or aboard ground vehicles, watercraft,or aircraft.

One function of the automated testing system is to measure the signallevel required to acquire service from a particular cell site 12. Toperform this test a monitoring site 10 goes “active,” transmitting on acellular channel control channel assigned to a cell site 12 within rangeof the monitoring site. The power level of the transmitted signal isinitially low enough that it is near or below the noise threshold of thecell site equipment, and thus is not recognized by the cell site. As thepower level is gradually increased, the signal reaches a level which isdetected by the cell site under test, which then begins the process ofestablishing the call. The power level required to acquire service isrecorded; the test is repeated periodically, so that comparison of testdata indicates any change, trend, or degradation in equipmentperformance. Also, of course, major failures such as a lack of responseby the cell site on one or more channels are immediately obvious.

Another function of the system is to test operation of hand-offs betweencell sites. A monitoring site can be instructed to establish a callthrough a particular cell site. With the call established, the transmitpower level from one monitoring site is decreased while the transmitpower level is increased at the other monitoring site. In advancedprotocols, the monitoring sites may also provide increasingly biased oroffset measures of signal quality. The effect is the same as though amobile station were traveling through the cellular network, with its RFlink to one cell site weakening while growing stronger and of betterquality to the other. When the system performs a hand-off, the time ofhand-off is recorded along with the power levels (and applicable signalquality reports) being transmitted to each cell site. The systemmethodically tests channels in use at each cell site, and may performseveral back-and-forth handoffs during each channel test. Such testinginitially establishes a baseline of data for comparison to future tests;as described above, comparison of periodic hand-off tests will indicateany changes in cell-site performance parameters. By handing the callback and forth between the cell sites, any bias in the hand-offprocedure can be detected and measured, as well as the speed of responseby each cell site.

In a passive mode, a monitoring unit at a cell site 12 scans cellularfrequencies and/or logical “channels”, looking for activity. A receiveris programmed to detect cellular channels for cell sites within themonitoring site's range; the receiver may be part of, or integratedinto, the test mobiles in the monitoring unit. Adjacent or nearby cellsmay not use the same cellular channels because such use would causeinterference. Many cell sites may be assigned the same cellularchannels, but those using the same channels must not be close enough tocause interference with each other's calls. When the test system detectsa received signal on a particular channel, the computer controllerdetermines whether a call is in progress on that forward channellocally. Lack of a local call indicates that some nearby cell is usingthat channel, increasing chances for interference. Data collected in themode just described is recorded for analysis.

Another function of the system is analysis of data to determine thelocation of a mobile station. FIG. 2 illustrates mobile telephone unitsplacing 911 calls within the cellular telephone system of an urban area.Given the density of cell sites, such a call is often received atseveral cell sites; a known relative-time-of-signal-arrival procedurewill be used to time-stamp the call. The same procedure is used tocollect time information from other cell sites which receive the call.Using the relative time-of-arrival information, a triangulationalgorithm is used to determine a position of the mobile station fromwhich the call is placed. In some regions shown in FIG. 2, monitoringsites not colocated with cell sites provide additional time-of-arrivaldata points necessary to provide a unique position solution or toenhance accuracy.

FIG. 3 is a schematic of a mobile telephone unit in a rural area. Widerspacing of cell sites precludes triangulation by signal time-of-arrivalas described above without the use on non-colocated monitoring sites 10,because the call is not received at a sufficient number of cell sites.According to the invention, however, the call will be received at one ormore monitoring sites 10 in addition to one or more cell sites 12. Insome regions of FIG. 3, these monitoring sites provide the majority ofthe data points needed for a unique position determination. Each monitorsite is configured to monitor control channels accessible by a mobileunit within the radio horizon of the monitor site. Time utilization ofreverse control channels is statistically low, therefore positiondetermination (time of arrival) data may be received by monitoringstations 10 even where the monitoring station 10 receives signals fromregions containing more than one cell site 12 reusing the same controlchannel(s). Because the cellular telephone system in the United Statesis structured so that each area is served by two cellular serviceproviders and possibly several PCS providers, the available controlchannels may include those of more than one provider, with potentialsharing of monitoring resources.

To locate the position of a mobile station, call access attempts aremonitored. A “call access attempt” by a mobile station, as used herein,includes a call origination, a response to a call, or a registration.Detection of preselected dialled numbers such as the emergency 911number, or preselected user identity numbers prompt the monitorcontroller to mark the arrival time of the signal, the ESN/MIN(Electronic Serial Number/Mobile Identification Number) of the callingunit, and the channel on which the call is received. Other informationrecorded includes the DCC or other parameters which identify the sitebeing accessed, and observable parameters such as angle of arrival. Themonitor controller then contacts a central computer, an E-911 computerdesignated to receive location information; this contact may be adial-up call (either land-line or cellular), a “connectionless” networklink, or via a dedicated circuit. With the contact established, theESN/MIN, time information, and observable signal parameters will bedownloaded to the E-911 computer. The information thus transmitted iscombined, by the E-911 computer, with the similar information receivedby the serving cell site and/or other monitoring sites.

Using a time-difference-of-arrival algorithm, the E-911 computerdetermines a geographic location of the mobile station from which thecall was placed. In an alternate embodiment, the information processingto determine a geographic location is performed by the computercontroller for the cellular telephone system; that is, the “E-911computer” is but a software package in another computer.

The present invention uses a frequency domain approach to determine thesignal time of arrival at various monitoring sites. This differs fromthe traditional time domain method, which involves performing a crosscorrelation on signals received a two or more sites. Correlation is donedirectly by the correlation integral, or by the Fourier transformmethod. (In the “Fourier Transform Method:, the two time domainfunctions are transformed to the frequency domain, then multiplied(Hermetian product) together, and the product is inverse transformed.)In either case (they are mathematically equivalent), a cross correlationfunction is generated; the position of the peak or maximum of the crosscorrelation function is related to the time shift between the twofunctions.

In this invention a frequency domain approach is used, in which thereceived signals from two different locations are decomposed into theirconstituent frequencies. The phase functions for the two signals arefound. The difference in these phase functions, when expressed as afunction of frequency, will ideally be a linear equation whose slope isdirectly proportional to the time difference of arrival. Likewise the“y” intercept will be the relative frequency error between the twosignals. Since there will likely be noise present, a linear regressionmay be performed to find the slope and intercept. Alternatively, aderivative of the phase function may be found. Ideally, it will have aconstant valued function of frequency that may be smoothed by filtering.This phase derivative approach offers an advantage of avoiding thediscontinuities that accompany the phase unwrapping associated withfinding the phase function.

Advantages of the frequency approach just described include: 1) avoidingthe computation of the cross correlation function and the subsequentpeak search; 2) allowing direct frequency domain filtering to optimizesignal-to-noise ratio, i.e., the signal power as a function of frequencymay be used to weigh the filtering; 3) a direct yield of the TDOA; 4)utilization of the efficient fast Fourier transform; and 5) using aneasy way to avoid the phase unwrapping problem associated with findingphase shifts.

It is understood that unique determination of a mobile station'sgeographic location requires data from three or more sites. In a ruralarea, data may be available only from a monitoring site and one basestation. Most location algorithms, given data from two sites, willprovide two possible locations for the mobile station placing the call.In most cases, however, these possible locations can be narrowed to onewhen the location data is superimposed on a map. If one locationcoincides with a road or a populated area while the other does not, theformer is the most likely location of the mobile station.

The restrictive description and drawings of the specific examples abovedo not point out what an infringement of this patent would be, but areto enable one skilled in the art to make and use the invention. Variousmodifications can be made in the construction, material, arrangement,and operation, and still be within the scope of our invention. Thelimits of the invention and the bounds of the patent protection aremeasured by and defined in the following claims.

We claim as our invention:
 1. A method for determining a geographicposition of a mobile telephone station in combination with automatictesting of cellular telephone equipment, comprising the following steps:locating a plurality of monitoring sites so that each said monitoringsite receives cellular-frequency transmissions within a geographicalarea including one or more cellular telephone base stations,electronically connecting said monitoring sites to a computercontroller, monitoring cellular-frequency control channels accessible bya cellular telephone mobile station within a radio horizon of saidmonitoring site, measuring, at said monitoring site, signal parametersof monitored transmissions from said mobile stations and from said basestations, and determining a position of a mobile telephone station bydetermining and analyzing the phase difference for a plurality ofcomponent frequencies of the signals received at two or more monitoringsites from said mobile telephone station.
 2. The method as described inclaim 1, wherein one or more said monitoring sites are colocated withcellular telephone base stations.
 3. The method as described in claim 1,wherein the step of determining a position of a mobile telephone stationfurther comprises: decomposing a signal received from a mobile telephonestation into said signal's component frequencies, determining amagnitude and a phase for a plurality of said component frequencies,determining, versus frequency, differences in phases for a plurality ofindividual frequencies received at two or more monitoring sites,measuring a slope of a phase-vs-frequency relationship for a pluralityof frequencies, said slope being directly proportional to a relativetime of arrival of said signal at differing monitoring sites, andcomputing from said time of arrival at different monitoring sites ageographic location estimate for said mobile telephone station.
 4. Themethod as described in claim 1, wherein the step of locating a pluralityof monitoring sites further comprises: locating at least one saidmonitoring site at a sufficiently high elevation so that said monitoringsite receives cellular-frequency transmissions within a geographicalarea including two or more cellular telephone base stations.
 5. Themethod as described in claim 1, wherein the step of monitoringcellular-frequency control channels further comprises: scanning apre-determined group of control channel frequencies using an automatedscanner.
 6. The method as described in claim 1, further comprising thefollowing steps: measuring a power level required to access service froma particular base station by transmitting from one said monitoring site,at a gradually increasing power level, on a control channel assigned tosaid base station, recording a transmitted power level at which saidbase station responds to a transmission from said monitoring site,repeating the measuring step and the recording step from time to time,and comparing recorded transmitted power levels from repeated measuringsteps to detect any change in a power level required to access servicefrom said base station.
 7. The method as described in claim 1, furthercomprising the following steps: measuring dynamic power-controlperformance of a particular cell site by establishing a test callbetween a monitoring site and said cell site, transmitting from saidmonitoring site at different power levels during said test call,recording a power level transmitted from said monitoring site at whichsaid cell site initiates a power-adjustment command, and repeating saidtransmitting step and said recording step for different levels of powertransmitted from said monitoring site.
 8. The method as described inclaim 1, further comprising the following steps: measuring a power levelrequired to access service from a particular base station bytransmitting from one said monitoring site, at a gradually increasingpower level, on a control channel assigned to said base station,recording a transmitted power level at which said base station respondsto a transmission from said monitoring site, repeating the measuringstep and the recording step from time to time, and comparing recordedtransmitted power levels to detect any change in said power levelrequired to access service from said base station.
 9. A method fordetermining a geographic position of a mobile telephone station incombination with automatic testing of cellular telephone equipment,comprising the following steps: placing a plurality of monitoring sitesat sufficiently high elevations so that each said monitoring sitereceives cellular-frequency transmissions within a geographical areaincluding one or more cellular telephone base stations, electronicallyconnecting each said monitoring site to a computer controller,monitoring cellular-frequency control channels accessible by a cellulartelephone mobile station within a radio horizon of said monitoring site,detecting a call access attempt from said mobile station, determining atime of arrival for said access attempt by analyzing the phases of aplurality of component frequencies obtained from the mobile stationtransmission, recording a time at which said call access attempt wasreceived at said monitoring site, recording an electronic identity ofsaid mobile station making said call access attempt, determining a basestation through which said call access attempt is routed, comparing saidtime at which said call access attempt was received at said monitoringsite to a time at which said call was received at said base station, andcomparing the recorded times to determine a position estimate of saidmobile station.
 10. The method as described in claim 9, wherein saidcall access attempt includes dialled digits “911”.
 11. The method asdescribed in claim 9, wherein the step of recording said electronicidentity of said mobile station includes recording other observablesignal parameters from said mobile station.
 12. The method as describedin claim 9, wherein the step of recording said electronic identity ofsaid mobile station includes recording a serial number (ESN) of saidmobile station.
 13. The method as described in claim 9, wherein the stepof recording said electronic identity of said mobile station includesrecording a mobile identification number (MIN) of said mobile station.14. The method as described in claim 9, further comprising the followingsteps: comparing said recorded times with a recorded time at which saidcall access attempt was received at a third cellular facility, andcalculating from the recorded times at which said call access attemptwas received at said monitoring site and at said base station and atsaid third cellular facility station to determine an exact location ofsaid mobile station.
 15. The method as described in claim 9, furthercomprising the following steps: comparing said possible position of saidmobile cellular telephone station with information on a geographic mapto determine a most likely position of said mobile station.
 16. A methodfor determining a geographic position of a mobile telephone stationcomprising the following steps: locating a plurality of cellularfacilities for transmitting signals to and/or receiving signals from amobile telephone station, wherein a portion of the radio coverage areaof at least one of said plurality of cellular facilities overlaps aportion of the radio coverage area of at least one other of saidplurality of cellular facilities; electronically connecting saidplurality of cellular facilities to a computer controller; receivingsignals from a mobile telephone station within a radio horizon of one ormore of said plurality of cellular facilities; and determining aposition of a mobile telephone station by determining and analyzing thephase difference for a plurality of component frequencies of the signalsreceived at two or more of said plurality of cellular facilities fromsaid mobile telephone stations.
 17. The method of claim 16 wherein theplurality of cellular facilities are selected from among a cellular basestation and a monitoring site having a radio coverage area overlapping aplurality of cellular base station coverage areas.
 18. The method ofclaim 17 further comprising measuring, at a cellular facility, signalparameters of monitored transmissions from said mobile telephonestations and from said cellular base station.
 19. A method fordetermining a geographic position of a mobile telephone station,comprising: locating a plurality of cellular facilities, fortransmitting signals to and/or receiving signals from a mobile telephonestation, wherein a portion of the radio coverage area of at least one ofsaid plurality of cellular facilities overlaps a portion of the radiocoverage area of at least one other of said plurality of cellularfacilities, and wherein said plurality of cellular facilities compriseone or more cellular telephone base stations and one or more monitoringsites, and wherein said one or more monitoring sites receive cellularfrequency transmissions within a geographic area including one or morecellular telephone base stations; electronically connecting each saidplurality of cellular facilities to a computer controller; monitoringcellular-frequency control channels accessible by a cellular telephonemobile station within a radio horizon of at least two of said pluralityof cellular facilities; detecting a call access attempt from said mobiletelephone station; determining a time of arrival for said access attemptby analyzing the phases of a plurality of component frequencies obtainedfrom the mobile station transmission; recording a time at which saidcall access attempt was received at said at least two of said pluralityof cellular facilities; recording an electronic identity of said mobilestation making said call access attempt; comparing said time at whichsaid call access attempt was received at said at least two of saidplurality of cellular facilities to determine a position estimate ofsaid mobile telephone station.
 20. The method as described in claim 19,further comprising the following steps: comparing said recorded timeswith a recorded time at which said cell access attempt was received at athird and/or additional cellular facilities; and calculating from therecorded times at which said call access attempt was received at said atleast two of said plurality of cellular facilities and said third and/oradditional cellular facilities to determine an exact location of saidmobile telephone station.